When water is freely available, animals working for small portions of food on certain variable-interval schedules typically drink far more than they would if food were freely available. This excessive drinking, called schedule-induced polydipsia, has inspired several theoretical accounts, but no explanation has yet gained wide acceptance. Models derived from conservation theory have been applied successfully to performance under a wide variety of schedules, including simple and complex ratio and interval schedules. The proposal describes a conservation model which may account for the occurrence of polydipsia with small portions of food, and its disappearance with larger portions. The major objective is an experimental test of the model's ability to fit the quantitative details of lever pressing, eating, and drinking in rats under conditions designed to produce varying degrees of schedule-induced polydipsia.